Direct conversion of alternating current (AC) electric power into reciprocating mechanical power by resonant motors, and the reverse conversion in alternators, has become important in applications like pulse-tube and Stirling-cycle cryocoolers and small externally-heated engine-generators operating on a thermoacoustic or Stirling cycle. Unlike more common rotary motors, the moving parts in such devices reciprocate, typically along the central axis of the assembly. The movement is typically guided with non-contacting bearings or non-rubbing flexures, enabling use of non-contacting and non-wearing close-clearance seals between pistons and cylinders. Such seals, though fully capable of adequately impeding alternating flow at operating frequency, nonetheless can allow one-way flow (leakage) if a suitable pressure difference arises across the seal. In practice, these pressure leakages arise due to geometric anomalies or asymmetric pressure-position relationships. Leakage leads to accumulation of excess gas on one side of the piston that pushes the piston toward the depleted side in a phenomenon called “drift.” Uncorrected drift leads the piston to move to the end of its allowed travel, limiting or preventing further reciprocation. The tendency to drift is proportional to the amplitude of the pressure wave in the device, which is a stronger-than-proportional function of the piston stroke. As a result, drift occurs minimally at low strokes, but becomes a severe problem at higher strokes.
In past practice, especially in free-piston Stirling engines, a feature called a “centerport” has been used to address leakage and piston mis-positioning. A centerport is a set of aligned ports in both the cylinder and moving piston of a free-piston device. The ports align when the piston is near its intended mid-stroke position. That position-dependent alignment of ports creates a momentary short-circuit or bypass of the piston seal. When the piston is not at its intended midstroke position, the ports are effectively blocked off or closed by the close fit of the piston clearance seal. For devices where the mean pressure and mean position are coincident in time, this arrangement provides a robust, passive correction for any drift that causes unequal pressure during port alignment. In this case, the undesired pressure difference drives a corrective gas flow. However, centerports are not ideal for all situations. For instance, they do not work well if there is a significant phase angle between pressure and motion, i.e. when a substantial pressure difference exists at the times when there is port alignment (in a centered piston position). In this case, the otherwise corrective flow of the centerport leads to a wasteful flow loss at the ports. Unfortunately, a large class of commercially significant machines exhibit such a phase shift, making centerport systems too inefficient for use with these machines.
Centerports also create at least some minimum, unavoidable loss for low-phase devices (e.g., free-piston Stirling engines) since there is always some phase difference. In addition, the required ports for low-phase devices are typically very small, precise orifices to avoid over-correction. These small orifices are susceptible to clogging, as well as being costly to manufacture. Centerports are also completely contained within the deepest parts of the free-piston device, which requires costly disassembly and/or part replacement if a malfunction occurs. Further, even without a discrete malfunction, there is no mechanism for adjusting centerports while in service to compensate for changing conditions in the seal or drift.
Another piston position or drift control practice provides an external circuit around the piston seal and at least one control valve in the circuit. Sensing means are employed to detect piston position. The piston position data is used, through a microprocessor control, to momentarily open the control valve to enable corrective flow when excess piston drift is detected. Often two active control valves are used in parallel in a network with a check valve before or after each control valve. In this case, each control valve is used to provide corrective flow in just one direction. This simplifies control algorithms and reduces the required duty cycle for the control valves. Such systems work well, but require extensive external, pressurized piping and valves, as well as costly position sensors and a controller. Such active systems are easily repaired, easily adjusted, and adapt without further intervention to changing conditions of seal flow and drift. However, the external plumbing is more susceptible to leakage and damage, and the increased complexity implies lower reliability.
Another piston position or drift control practice provides a tuned acoustic waveguide bypass around the piston seal, presenting high alternating flow impedance (and therefore little loss on the seal function), but low unidirectional flow impedance (therefore presenting little restriction to corrective flow that keeps mean pressures equal across the piston seal). An acoustic bypass can be built internally or externally, and consists of a long, narrow passage (e.g., a tube) between internal volumes of the device. The length of the bypass is many times its flow area and ideally substantially equal to a one-half wavelength (or multiple thereof) of the free-propagation of sound in the sealed medium of the device at the frequency of piston reciprocation. This type bypass is passive like centerports, but without the complex, precision machining required for centerports. However, the acoustic bypass is sensitive to operating frequency. In addition, an acoustic bypass is difficult to apply efficiently due to actual gas flow losses near the ends of the tube unless the drift to be corrected is very slight. Accordingly, this practice is generally suitable only for devices with extremely good seals or little penalty for lower efficiency.
In view of the foregoing, there is a need in the art for an improved piston position drift control and a related free-piston device using the same